In mobile communication, as represented by cellular phones, attempts have hitherto been made to lengthen the standby period of the mobile terminal by saving its battery power consumption, and a number of techniques have been developed.
Japanese Patent Publication 2000-174689, for example, discloses a technique in which the portion of a mobile terminal which is not currently operating during a standby mode is set in a power-saving mode or completely deactivated. When the mobile terminal is out of the range of a cell site, it controls the rate at which it receives control data from the network according to the field strength of the receiving signal. The rate of this intermittent mode of reception is also controlled according to the remaining battery power.
In addition, Japanese Patent Publication 10-84572 discloses a technique in which the mobile terminal controls the rate at which it scans the spectrum for signals from the network during a frequency (cell) search according to the amount of time lapsed from the instant it started the cell search. If the lapsed time becomes overly long, mobile terminal reduces the scan rate for saving battery power. The scan rate is also controlled according to the amount of power remains in the battery.
Despite the prior art power saving techniques, the current tendency is toward making a further effort for developing smaller and lighter mobile units with an attendant effort for reducing the size of batteries. On the other hand, the lengthening of the standby period is still in demand. The present invention is intended to achieve battery saving of a mobile terminal while it is within the range of a cell site and is operating in a standby mode, rather than to achieve battery saving of a mobile terminal located in out-of-the-range areas, unable to find a cell site.
The present invention relates to mobile terminals of a CDMA communications system in which the mobile terminals are designed to receive RF signals of different frequencies from a given network or from different networks to monitor their field strengths so that the mobile terminals can operate in an optimum standby mode. More specifically, when the mobile terminal is in a border area of two networks and establishing a communication with a cell-site or in a standby mode using a certain frequency channel, it receives a broadcast command signal from the cell-site. This broadcast signal contains information that specifies frequencies used by other cell sites of the border area. In response, the mobile terminal makes a search for the specified frequencies and detects their field strengths.
The operation of a frequency monitor (or search) requires the mobile terminal to retune its receiver from one frequency to the next. When the receiver is retuned, the channel is switched from one active circuit to an inactive circuit, which takes time to activate. Thus, the time taken to scan across the frequency spectrum is substantial and the amount of power dissipated is greater than is required to monitor a single frequency.